This invention relates to an assembly which prevents the falling of fine dust from the roof and the break in a mining operation. The assembly includes the shield support and the step support and prevents existing stone dust and stone or rock from falling from the roof and the break.
The amount of stone dust around a shield support in the working region of a mining operation is relatively large. The prior art has provided a shield support and a step support which is used to prevent the roof of a mine from caving in. The shield or frame supports include roof bars or caps which are pressed under the roof with a great amount of pressure. The shields or frames are moved forwardly as the mining operation continues. The forward movement of the shield or frame causes an alternation of stress on the roof thereby forming a layer of loose stone above the roof bars. This is known in the prior art as the "trample effect". The loose stone layer is described as debris and comprises pieces of stone of varying size. The proporation of dust is relatively low in this debris.
It is further known to fit a roof covering between the roof and the roof bars of the shield or support frames to further prevent this debris from falling through the spaces between the roof bars into the working region therebelow. The prior art roof covering usually consists of a wire-netting sheets which extend from the forward points of the roof bars at the face over the roof bars into the break. The wire-netting sheets can be arranged both parallel to the face of the roof bars and also in the working direction which is transverse to the face. When the shield or frame supports are moved forwardly during the mining operation, the wire-netting sheet is unrolled from a roll held in readiness. Alternatively, a sheet may be fitted to another sheet so that the entire roof surface is covered with wire-netting. Any debris loosened above the wire-netting is held on the roof. Upon movement of the shield or frame supports as noted, the debris is carried off to the break at a point displaced from the working area.
The shield supports and the shields are closely adjacent one another. However, upon moving the shields or the supports, the debris may pass into the gaps between the shields even if wire of the conventional type is already drawn thereacross. That is, part of the debris may fall through the mesh of the wire-netting. When parts of the debris fall between the shields, the reciprocal movement of the shield grind the stone to dust therebetween. The dust is whirled up by the air current and thus passes into the respiratory systems of the miners.
Heretofore, the roof and coal face of the mine has been sprayed with relatively large amounts of water to hold the amount of dust down. This is a disadvantage because the water binds the existing dust particles which coagulate to a cement-like mass and restrict the freedom of movement of the shield roof bars. The cement-like mass particularly obstructs the triangular shaped space between the shield roof bar and the break shield. Consequently, the shield roof bar becomes almost incapable of movement.
It is known that the dust which arises when the debris occurs above the roof bars as a result of the trample effect is still relatively small. The greatest amount of dangerous dust is produced only when those shields and their supports are moved.